Aloeresin H, a new polyketide constituent of Cape aloe

Article abstract of DOI:10.1016/S0040-4020(02)01488-6

A new constituent, aloeresin H (1), was isolated from Cape aloe, a bittering and laxative agent. Its structure was elucidated by degradation experiments combined with 1D and 2D NMR data. Aloeresin H represents the first C,C-diglucoside discovered in commercial samples of the drug and its polyketide origin can be interpreted in terms of two-chain condensation. Possible conformations of the virtual aglycone were obtained as energy minima by quantum mechanical calculations and were found to be consistent with particular NOE associations observed in the original glucoside.

Full text of DOI:10.1016/S0040-4020(02)01488-6

Tetrahedron 59 (2003) 401–408
Aloeresin H, a new polyketide constituent of Cape aloeq
a
b
c
Paolo Manitto,a Giovanna Speranza, Nunziatina De Tommasi, Emanuele Ortoleva
,
,
*
*
and Carlo F. Morellia
a
`
Dipartimento di Chimica Organica e Industriale, Universita degli Studi di Milano, via Venezian 21, I-20133 Milano, Italy
Dipartimento di Scienze Farmaceutiche, Facolta di Farmacia, Universita degli Studi di Salerno, via Ponte Don Melillo,
b
`
`
Invariante 11C, I-84084 Fisciano (SA), Italy
`
Dipartimento di Chimica Fisica ed Elettrochimica, Universita degli Studi di Milano, and Istituto CNR, ISTM,
via Golgi 19, I-20133 Milano, Italy
c
Received 14 September 2002; revised 23 October 2002; accepted 14 November 2002
Abstract—A new constituent, aloeresin H (1), was isolated from Cape aloe, a bittering and laxative agent. Its structure was elucidated by
degradation experiments combined with 1D and 2D NMR data. Aloeresin H represents the first C,C-diglucoside discovered in commercial
samples of the drug and its polyketide origin can be interpreted in terms of two-chain condensation. Possible conformations of the virtual
aglycone were obtained as energy minima by quantum mechanical calculations and were found to be consistent with particular NOE
associations observed in the original glucoside. q 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
2. Results and discussion
The drug known as Cape aloe, which is the dried exudate
from the cut leaves of Aloe ferox Miller (fam. Liliaceae),1 is
widely used for its cathartic properties2 and as a bittering
agent in alcoholic beverages.3 It has been shown to be a
complex mixture of secondary metabolites,4 synthesized
through the acetate-malonate pathway,5 as well as of a small
number of process products.4b,6 The main natural products
so far isolated amount to 50–60% in weight of the drug
and belong to the biogenetic families of hexaketides
(e.g. 1-methyltetralins),7 heptaketides (e.g. 5-methylchro-
mones),8a–foctaketides (e.g. 1,8-dihydroxy-9-anthrones),9a
and nonaketides (3-benzylisocoumarins).10 An additional
feature of these compounds is the frequent C-glucosyl-
ation,4a aloesin (4, formerly aloeresin B)8a,c being a
representative in this regard.
Aloeresin H (1) was obtained as an amorphous pale yellow
powder from EtOAc–MeOH–CHCl3 extracts of different
commercial samples of Cape aloe (ca. 0.1% yield based on
the solid drug) by means of combined chromatographic
techniques.
In continuation of our chemical investigation into Cape aloe
we report here the structural elucidation of a new constituent
we named aloeresin H. This name was suggested by the
presence in the molecule of a 8-C-b-D-glucopyranosyl-7-
hydroxy-5-methylchromone portion (cf. 4) which is
common to all aloeresins (A–G) described so far.4–8a–f,i
Aloeresin H (1) represents the first C,C-diglucosylated
molecule discovered in Aloe spp. exudates or extracts.
Its positive- and negative-ion FABMS spectra indicated a
molecular weight of 770 Da, and the corresponding
molecular formula C38H42O17 was deduced as follows. By
treatment with dimethyl sulfate aloeresin H (1) furnished
a tetra-O-methyl derivative (2) which in turn gave an octa-
acetate (3) by acetylation with Ac2O/Py (pseudomolecular
q Part16 in the series ‘Studies on Aloe’. Part 15, see Ref. 6b.
Keywords: aloe; polyketide; 5-methylchromones; conformation; quantum
mechanical calculations.
*
Corresponding authors. Tel.: þ39-25031-4098; fax: þ39-25031-4072;
e-mail: paolo.manitto@unimi.it
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)01488-6

402
P. Manitto et al. / Tetrahedron 59 (2003) 401–408
Table 1. NMR data of aloeresin H (1) and its tetramethyl ether (2) in
DMSO-d6 at 600 MHz (1H) and 150 MHz (13C)
differential NOE experiments performed on aloeresin H
(1) and on its tetra-O-methyl ether (2) supported the
existence of the molecular fragments represented in
Figure 1. An additional singlet at d 5.98 and 6.07 in the
1H NMR spectra of 1 and 2, respectively, could be assigned
to a fifth olefinic proton (see Table 1 for the corresponding
13C chemical shifts). Evidence for a methylene group
having readily exchangeable protons was provided by the
disappearance of the signals of an AB-system in the
spectrum of 1 recorded in DMSO-d6 (see Table 1) and in
acetone-d6 (dA¼4.52, dB¼4.04, J¼16 Hz) after D2O
addition. The APT spectrum of aloeresin H (1) in DMSO-
d6 (see Table 1) displayed two signals characteristic of
carbonyl groups (d 178.50 and 202.8) and fifteen signals of
quaternary sp2-carbon atoms (including those of fragments
A, B and C shown in Fig. 1). Considering that four enolic
functions were demonstrated by the conversion of 1 into a
tetra-O-methyl ether (2)-but six oxygen-bearing quaternary
carbons were apparent in the d 150–160 of the APT
spectrum-an ethereal oxygen had to be added to the C, H, O
atoms previously identified in order to account for the
molecular formula of the aglycone.
No. C
1
2
dH (J, Hz)
dC
dH (J, Hz)
dC
2
3
159.58
114.30
178.50
114.97
140.20
22.16
160.08
113.94
178.44
116.25
141.89
22.53
112.72
161.84
56.28
5.98 s
6.07 s
4
4a
5
5-Me
2.71 s
6.70 s
2.76 s
6.96 s
6
116.50
159.30
7
7-OMe
3.89 s
8
8a
9
10
10-OMe
11
12
12-Me
13
14
15a
15b
16
17
18
18-OMe
19
20
20-OMe
21
22
110.50
155.52
118.32
155.30
113.54
157.19
120.26
159.75
56.28
111.07
141.01
21.18
3.81 s
6.92 s
6.72 s
114.80
140.20
20.60
122.80
135.20
48.21
2.28 s
6.58 s
2.32 s
6.72 s
123.18
134.56
48.60
4.27 d (17.2)
4.09 d (17.2)
4.15 brs
3.61 s
202.80
120.54
155.52
202.89
128.58
158.01
63.34
119.40
160.20
55.93
110.63
135.68
18.41
72.66a
70.56b
78.99c
70.10b
81.48d
61.02e
109.40
157.32
3.72 s
6.59 s
6.14 s
109.70
136.30
18.97
73.42
70.51
78.33
70.20
81.12
60.02
22-Me
10
1.88 s
1.95 s
4.72 d (9.8)
4.71 d (9.7)
3.81 dd (9.7)
3.13–3.20 m
3.25–3.32 m
3.13–3.20 m
3.56 br d (11.4)
3.67 br d (11.4)
4.68 d (9.6)
3.47 dd (9.6)
3.25–3.32 m
3.25–3.32 m
3.25–3.32 m
3.56 br d (11.4)
3.67 br d (11.4)
f
20
f
f
f
f
300
4
50
60
Figure 1. Structure fragments of aloeresin H (1) and of its tetra-O-methyl
ether (2) as identified by spectroscopic data (see Table 1 for d assignments).
100
200
300
400
500
600
74.79
71.93
77.77
69.45
81.06
60.02
4.67 d (10.0)
f
73.37a
70.56b
78.52c
70.77b
81.00d
61.64e
f
f
f
f
A number of spectroscopic data of aloeresin H (1) were
reminiscent of the 7-hydroxychromone system (Table 2).
In particular, the electronic spectrum resembled that of
aloesin (4) both in its maxima and in the corresponding
extinction coefficients, except for the broad absorption band
at higher wavelength which appeared to be more prominent
than in aloesin. In addition, the chemical shift values of the
olefinic proton at the highest field, of the methyl protons at d
2.71, and of the other atoms of the fragment A in Figure 1
(Table 1) were very similar to those of the 7-hydroxy-5-
methylchromones occurring in Cape aloe.8a–f The signifi-
cant low field resonance of the methyl group could be
attributed to the deshielding effect of the carbonyl
group (dC¼178.50, n¼1651 cm21) of the chromone
nucleus.8a –g The indication of a 7-hydroxy-5-methyl-
chromone as a molecular portion connected to the remnant
of the molecule through the carbon-2 was corroborated by a
chemical correlation between aloesin (4) and aloeresin H (1)
(Fig. 2). It was found that 7-O-methylaloesin pentaacetate,
predominantly in the 9Z-configuration (5) as shown by its
NMR data, and tetra-O-methyl aloeresin H octaacetate (3),
when subjected to ozonization followed by CrO3-oxidation,
gave the same degradation product 6, whose structure was
confirmed by spectroscopic data.
a–e
Signals of glucose hydroxyl groups were observed in the range 3.0–5.0
d and for 1 broad signals of phenolic groups in the range 9.5–10.5 d.
Signals with the same superscripts are interchangeable.
f
Overlapping signals in the range 3.0–4.4 d.
ions at m/z 827 and 1163, respectively, in positive FABMS).
Mono- and bi-dimensional NMR analyses of aloeresin H (1)
revealed the presence of two C-b-D-glucopyranosyl resi-
dues. Proton and carbon-13 signals due to either one of the
1
residues were distinguished by H–1H COSY, HMQC and
TOCSY11 experiments (see Table 1).
The values of 1H and 13C chemical shifts and 1H–1H
coupling constants belonging to each set of resonances were
consistent with two glucose units,12 both attached to an sp2
carbon atom through the b-bond (JHH ca. 9.7 Hz)13 of the
anomeric carbon (dH 4.68 and 4.71; dC 74.79, 73.42).14
Therefore, the molecular formula C26H22O7 could be
assigned to the aglycone (two hydrogen atoms replacing
the glucosyl residues). COSY, HMQC, HMBC and

P. Manitto et al. / Tetrahedron 59 (2003) 401–408
403
Table 2. UV data (lmax in nm, log 1 in parentheses) of aloeresin H (1) and selected reference compounds
EtOH
0.01 M aqueous NaOH
Aloeresin H (1)
230 infl.
(4.37)
244
(4.29)
246
254
(4.25)
254
299
(4.22)
298
260
(4.47)
265
320 infl.
(4.25)
316
344
(4.43)
Aloesin (4)a
390b
(4.32)
(4.17)
242
(4.10)
242
(4.24)
(4.16)
250
(4.12)
249
(4.32)
(391)
295
(3.97)
296
(4.04)
(4.25)
258
(4.31)
(3.93)
320
(3.97)
7-Hydroxy-2,5-dimethylchromonec
7-Hydroxy-2-methylchromoned
Resacetophenone (10)e
Orcacetophenone (11)f
231
278
315
256
(3.90)
336
(3.93)
233
(3.88)
(4.14)
282
(3.99)
(3.91)
320
(3.78)
(4.42)
330
(4.07)
An intense band was present at lmax 210–230 nm (log 1 4.1824.54) in all spectra.
a
b
c
d
e
f
8a
Lit.
l
max
(EtOH) (log 1) 244 (4.14), 252 (4.18), 296 nm (3.90).
Presumably due to enolization of the acetonyl side chain.
Synthesized8a and isolated from Cape aloe.8f
Ref. 8h
Lit.17b lmax (pH 3) (log 1) 228 (3.97), 275 (4.14), 312 nm (3.83); (pH 11) 248 (3.81), 328 nm (4.41); cf. also Ref. 16b,17a,c,d.
Refs. 16b,17a,e.
The presence of a second C-glucosylated aromatic ring
incorporating the fragment (B) of Figure 1 was established
by isolation of compound 7 from the reaction mixture of
ruthenium-mediated oxidation15 of aloeresin H tetra-O-
methyl ether octaacetate (3) (Fig. 2). Structure 7 resulted
from spectroscopic analyses (see NOE associations in
Fig. 2); it was further substantiated by comparison of
UV16a,b and NMR16c data with those reported for 2,4-di-
methoxyphenylglyoxylic acids 8 and 9 (see Section 4).
That the two carbon atoms of the side-chain of 7
corresponded to a –CO–CH2-group in the original
molecule before oxidative degradation (the carboxylic
function of 7 arising from the methylene group) was
inferred by a number of spectral analogies between
aloeresin H (1) and 2,4-dihydroxyacetophenones. In fact,
the UV spectra of aloeresin H (1), resacetophenone (10) and
orcacetophenone (11) in basic solution all display a broad
band at 344, 336 and 330 nm, respectively, due to the
marked bathochromic shift of the maxima in the range 280–
300 nm, exhibited by these compounds in ethanol solution
(Table 2). Furthermore, the 13C chemical shift of the non-
chromone carbonyl group of aloeresin H (dC¼202.80) and
the frequency of its IR absorption (broad band centered at
1605 cm21 including phenyl ring absorption) appear similar
to those shown by the acetophenones 1017d,g and 11,17f
which reflect the characteristic chelation occurring in
o-hydroxy aromatic ketones and aldehydes.18 To account
1
for all H and 13C NMR data (Table 1) and the molecular
formula, a third benzene ring having the four contiguous
carbon atoms of the fragment (C) (Fig. 1) and the two
remaining quaternary carbons at d 118.32 and 135.20 had to
be inserted into the molecule of aloeresin H (1).
In agreement with a biogenetic explanation (see infra), the
most reasonable assemblage of the three identified blocks of
atoms, each containing an aromatic ring, leads to the
formula 1 for aloeresin H. This structure was definitely
confirmed by HMBC and NOESY experiments summarized
in Figure 3. Among them, the three-bond correlations of H-3
with C-9 and of H-13 with C-15 are of particular relevance
to define the connections between the aromatic rings.
Figure 2. Oxidative degradations of 7-O-methylaloesin pentacetate (5)
and aloeresin H tetra-O-methyl ether octaacetate (3). Significant NOE
associations are indicated. (a) Me2SO4, K2CO3, DMF–acetone; (b) Ac2O,
Py; (c) O3, CH2Cl2; (d) Jones reagent; (e) RuO2·nH2O, NalO4, CCl4–
CH3CN–H2O (3:3:4).
The particular NOE associations indicated in Figure 3 can

404
P. Manitto et al. / Tetrahedron 59 (2003) 401–408
These conformations differ almost exclusively in the sign of
the dihedral angle C3–C2–C9–C14 (see Table 3 in Section
4). In both of them the relative chemical shifts of the two
methylene protons involved in NOE associations (Fig. 3 and
Table 1) are also consistent with their position relative to
the the adjacent aromatic ring. Calculated dihedral angles
C9–C14–C15–Ha and C9–C14–C15–Hb are ca. 18 and
21188, respectively, in both conformers. Consequently, Ha
is almost coplanar with the m-cresol ring and therefore more
deshielded than Hb. It can be pointed out that the curvatures
of the minima are extremely low: normal mode analysis
reveals that low vibrational frequencies (about 10 cm21 or
less) are associated to torsional movements around C14–
C15 and C15–C16. These findings suggest a high flexibility
of the molecule with regard to the connection between the
2-arylchromone and orcacetophenone moieties.
Figure 3. Relevant HMBC and NOE correlations of compound 1 in
acetone-d6at 600 MHz (additional NOE correlations are indicated in
Fig. 1).
3. Origin of aloeresin H
be explained considering the minimum energy structures
generated for the virtual aglycone of aloeresin H by
quantum mechanical calculations using the program
Gaussian 9419 at the semi-empirical level AM1.20 Substi-
tution of hydrogen atoms for the two glucose residues (at
C-8 and C-19) was assumed to be of negligible influence on
the conformational preferences of the molecule core. This is
a major assumption, but it seemed to be justified under the
circumstances. On the other hand, such a substitution
allowed computational efforts to be greatly reduced. Eight
minima were found, having energy differences within
1.5 kcal mol21 (see Table 3 in Section 4). Among them,
the two conformations pictured in Figure 4 appear to be the
most favourable for 1H NOE associations across the
methylene group and the aromatic nuclei.
The formation of the aloeresin H molecule can be
rationalized in terms of intra- and intermolecular aldol
condensations (followed by dehydration) involving two
identical decarboxylated heptaketide chains4b,5 one of
which can be replaced by aloesin itself (4) in Figure 5. In
any case, this hypothesis accounts for the hydroxylation
pattern of the aromatic nuclei as well as for the position of
the three methyl groups, of the two C-glucosyl residues, and
of the ketomethylene C2-unit connecting the meta-cresol
and the resorcinol rings. Aloeresin H (1) could be a process
product resulting from thermal condensation reactions
between two aloesin molecules (4) with concomitant
opening of the g-pyrone ring of one of them. It should be
pointed out that the commercial drug is prepared by heating
Figure 4. Representative structures of the global minima calculated for the aglycone of aloeresin H (aloeresin H has two glucosyl residue at C-8 and C-19).
Relevant NOESY correlations and possible hydrogen bonds are indicated (cf. Tables 3 and 4 for geometry and energy data).

P. Manitto et al. / Tetrahedron 59 (2003) 401–408
405
`
Dipartimento di Chimica Organica e Industriale, Universita
di Milano.
4.3. Extraction and isolation
Powdered Cape aloe (1 kg) was extracted with EtOAc–
CHCl3–MeOH, 10:1:2 (10 L) with vigorous mechanical
stirring for two days at room temperature. After filtration,
the insoluble material was taken up with MeOH (1 L),
stirred for 3 h, filtered and evaporated under reduced
pressure to give a brown syrup (120 g). 40 g of this residue
was adsorbed on sea sand and fractioned by flash
chromatography (silica gel, 1.5 kg) eluting with EtOAc
containing increasing amount of MeOH. Separation was
monitored by TLC. Fractions containing aloeresin H (Rf
0.36) were combined, concentrated (5 g) and further
purified by flash chromatography (silica gel, 450 g) eluting
with EtOAc–EtOH–H2O, 100:20:13. Fractions were com-
bined on the basis of TLC analysis and evaporated to
dryness. The residue (ca. 700 mg) was chromatographed
over a Sephadex LH-20 column (250 mL) eluted with
MeOH–H2O 1:1 to give aloeresin H (300 mg) as an
amorphous powder, pure by TLC and analytical HPLC (tR
15.4 min); mp 237–2388C; [a]2D0¼þ29.3 (c 0.5, MeOH);
UV, see Table 2; CD (MeOH) lmax (D1) 230 (20.76), 248
(þ0.32), 340 (þ0.15); IR nmax (KBr) 1651, 1605, 1499,
Figure 5. Possible origin of aloeresin H through condensation of two
polyketide units.
for a few hours on an open fire the juice collected from the
cut leaves of Aloe plants.4b
4. Experimental
4.1. General procedures
Melting points were determined on a SMP3 apparatus
(Stuart Scientific) and are uncorrected. Microanalyses were
obtained with a Perkin–Elmer 240 elemental analyzer.
Optical rotations were measured with a Perkin–Elmer 241
polarimeter. UV spectra were recorded on a Hewlett
Packard 8452A Diode Array Spectrophotometer, CD
spectra on a Jasco J-500 instrument, and IR spectra on a
Perkin–Elmer FT-IR 1725 X spectrometer. 1H and 13C
NMR spectra were acquired at 599.19 and 150.858 MHz on
a Bruker DRX-600 spectrometer using the UXNMR soft-
ware package and at 300.135 and 75.469 MHz on a Bruker
AC 300 equipped with an ASPECT 3000 data system.
Chemical shifts (d) are given in ppm and were referenced to
the solvent signal: dH 2.50, dC 39.5; dH 3.30, dC 49.0; dH
2.05, dC 30.5 and 205.1 from TMS for DMSO-d6, CD3OD
and acetone-d6, respectively. FAB MS spectra were
recorded on a VG 7070 EQ mass spectrometer. Analytical
TLC was performed on silica gel 60 F254 aluminum sheet
(Merck) using EtOAc–EtOH–H2O, 100:20:13 as eluent,
unless otherwise stated. Components were detected by
spraying with 0.5% Fast Blue B salt, followed by heating at
1408C; preparative TLC on pre-coated silica gel 60 F254
plates (1 mm layer thickness, Merck). HPLC was carried out
on a Waters Model 600 E liquid chromatograph connected
to a HP 1050 Diode-Array detector (Hewlett–Packard);
chromatographic conditions: column, LiChrospher 100
RP-18 (5 mm, 125£4 mm, Merck); flow rate, 1 mL/min;
detector, l 225 nm; mobile phase, MeOH–H2O, linear
gradient from 30 to 90% MeOH in 30 min. Silica gel 60,
63–200 mm and 40–63 mm (Merck) was used for column
and flash chromatography, respectively. Aloesin was
isolated from Cape aloe as reported in Ref. 8d. Resaceto-
phenone was from Aldrich.
1454, 1384 cm21 1H and 13C NMR data, see Table 1.
,
FABMS m/z (positive mode) 771 [MþH]þ, 793 [MþNa]þ;
(negative mode) 769 [M2H]2. Anal. calcd for C38H42O17:
C, 59.22; H, 5.49. Found: C, 58.91; H, 5.55
4.3.1. Aloeresin H tetra-O-methylether (2). A solution of
aloeresin H (1) (100 mg, 0.13 mmol) in dry DMF (4 mL)
was diluted with dry acetone (15 mL) and treated succes-
sively with anhydrous K2CO3 (500 mg) and dimethyl
sulfate (0.2 mL, 2.1 mmol). The mixture was stirred at
room temperature under N2 monitoring the reaction
progress by TLC (t-BuOH–MeCN–H2O, 50:44:6). After
18 h, H2O (15 mL) was added and the solution heated on a
boiling water bath for 20 min, then cooled to room
temperature and neutralized with 0.5 M HCl. Removal of
the solvent under reduced pressure gave a residue which was
dissolved in the minimum amount of water (ca. 5 mL) and
desalted by passing through a column of Amberlite XAD-7
(wet volume, 75 mL) eluting with MeOH. Final purification
by column chromatography (silica gel, 15 g, AcOEt–
EtOH–H2O, 100:20:5) gave 2 as an amorphous powder
(92 mg, 85% yield), pure by TLC (eluent as above, Rf 0.42)
and analytical HPLC (tR 16.8 min); mp 264–2658C; UV
(MeOH) lmax (log 1) 216 (4.57), 226 (4.58), 244 (4.50), 250
(4.47), 294 (4.30) nm; IR nmax (KBr) 1647, 1600, 1569,
1458, 1376 cm21 1H and 13C NMR data, see Table 1.
,
FABMS m/z (positive mode) 827 [MþH]þ, 849 [MþNa]þ;
(negative mode) 825 [M2H]2. Anal. calcd for C42H50O17:
C, 61.01; H, 6.10. Found: C, 61.33; H, 6.02.
4.3.2. 7-O-Methylaloesin pentaacetate (5). 7-O-Methyl-
aloesin8i (100 mg, 0.24 mmol) prepared from aloesin (4)
following the experimental procedure reported above for
aloeresin H, was dissolved in dry pyridine (5 mL), cooled to
08C and treated dropwise with acetic anhydride (10 mL)
under N2. The reaction mixture was allowed to warm to
room temperature, stirred for 1 h (TLC control), and then
4.2. Drug samples
Commercial Cape aloe used in this investigation
was purchased from D. Ulrich Spa (Nichelino, Italy).
It was produced in the Port Elisabeth region (Cape
Town, South Africa). A voucher specimen is kept at the

406
P. Manitto et al. / Tetrahedron 59 (2003) 401–408
poured into 100 mL of ice-cold H2O and extracted with
CH2Cl2. The combined organic extracts were washed
successively with water, 0.5 M HCl, saturated NaHCO3
and with water again, and then dried over Na2SO4. Removal
of the solvent under reduced pressure and purification by
column chromatography (silica gel, 15 g, AcOEt–MeOH,
8:2) yielded 5 (130 mg, 87% yield) as a yellow crystalline
solid. TLC, Rf 0.88; mp 80–828C; 1H NMR (CD3OD,
300 MHz) d 1.68, 1.98, 2.03, 2.05 (4£3H, 4£s, 4£
CH3COO), 2.21 (3H, br s, CH3-11), 2.79 (3H, s, ArCH3),
3.98 (3H, s, OCH3), 4.08–4.35 (3H, m), 5.07–5.40 (3H, m)
and 5.83 (1H, m) (Glc protons), 6.05 (1H, br s, H-9), 6.43
(1H, br s, H-3), 6.91 (1H, s, H-6); relevant NOE
associations, see Figure 2; 13C NMR (CD3OD, 75 MHz) d
20.48, 20.94, 21.30, 21.58 (CH3COO and CH3-11), 23.77
(ArCH3), 57.38 (OCH3), 63.70 (CH2-60), 69.52, 70.35,
71.53, 76.39, 77.55 (CH of Glc), 110.68, 111.92 and 133.21
(C-3, C-6 and C-9), 111.00 (C-8 and C-4a), 145.55 (C-5),
157.57, 159.00 and 160.42 (C-2, C-7 and C-8a), 158.14
(C-10), 169.35 (CH3COO-10), 171.24, 171.77, 172.06 and
172.64 (CH3COO of Glc), 182.17 (s, C-4); FABMS m/z
(positive mode) 619 [Mþ1]þ. Anal. calcd for C30H34O14: C,
58.25; H, 5.54. Found: C, 57.77; H, 5.39.
mode) 1161 [M2H]2. Anal. calcd for C58H66O25: C, 59.89;
H, 5.72. Found: C, 59.51; H, 5.46.
4.3.4. Ozonization of tetra-O-methyl aloeresin H octa-
acetate (3) and of 7-O-methylaloesin pentaacetate (5). A
solution of 3 (100 mg, 0.086 mmol) in CH2Cl2 (10 mL) was
ozonized at 2608C until saturation (blue color, ca. 15 min)
and then purged with a stream of nitrogen to remove the
excess ozone. Cautious removal of the solvent under
reduced pressure gave a residue which was dissolved in
ice-cold acetone (5 mL) and treated dropwise with Jones
reagent until persistence of a brown color. The mixture was
then diluted with water (5 mL) and extracted with CH2Cl2.
The organic extract was washed with water, dried (Na2SO4)
and evaporated under reduced pressure. The residue was
purified by preparative TLC (EtOAc–MeOH, 9:1) to give
5.0 mg of 6 (11% yield). TLC, Rf 0.59; UV (MeOH) lmax
218, 252, 296 nm; 1H NMR (CD3OD, 300 MHz) d 1.72 (3H,
s), 1.96 (3H, s), 2.03 (6H, s) (4£CH3COO), 2.59 (3H, s,
ArCH3), 3.84 (3H, s, OCH3), 3.87 (1H, m, H-50), 4.12 (1H,
dd, J¼11.9, 2.2 Hz, H-6b0 ), 4.21 (1H, dd, J¼11.9, 4.8 Hz,
H-6a0 ), 4.98–5.20 (2H, m, H-10, H-40), 5.25 (1H, dd,
J0¼J00¼9.6 Hz, H-30), 5.96 (1H, dd,J0¼J00¼9.6 Hz, H-20),
6.30 (1H, brs, Ar-H); 13C NMR (CD3OD, 75 MHz) d 20.61,
20.66 (CH3COO), 24.23 (ArCH3), 56.07 (OCH3), 63.87
(CH2-60), 70.42, 71.19, 73.52, 76.80, 77.20 (CH of Glc),
106.23 (CH), 108.23, 145.76, 162.10, 163.99 (aromatic C),
171.38, 171.45, 171.84, 172.59 (CH3COO), 176.46
(COOH); FABMS m/z (positive mode) 535 [MþNa]þ;
(negative mode) 511 [M2H]2. Anal. calcd for C23H28O13:
C, 53.91; H, 5.51. Found: C, 54.15; H, 5.47. This compound
was found to be identical to the product obtained by
ozonization of 5 under the same reaction conditions.
4.3.3. Tetra-O-methyl aloeresin H octaacetate (3).
Acetylation of tetra-O-methyl aloeresin H (2) was carried
out as described above for 7-O-methylaloesin. After column
chromatography (AcOEt–MeOH, 8:2) compound 3 was
obtained (90% yield), pure by TLC (AcOEt, Rf 0.59) and
analytical HPLC (tR 19.5 min); mp 110–1128C; UV
(MeOH) lmax (log 1) 208 (4.76), 252 (4.41), 300
(4.28) nm; IR nmax (KBr) 1755, 1652, 1600, 1571, 1458,
1372 cm21, 1H NMR (CD3OD, 300 MHz) d 1.68 (6H, brs),
1.96 (15H, brs), 2.02 (6H, s) (8£CH3COO and CH3-22),
2.42 (3H, s, CH3-12), 2.83 (3H, s, CH3-5), 3.59 (3H, brs),
3.79 (6H, s), 3.96 (3H, s) (4£OCH3), 3.72–4.33 (10H, m,
Glc protons), 5.07 (1H, brt, J¼9.6 Hz), 5.87 (1H, brt, J¼
9.6 Hz) (H-20 and H-200), 5.21 (1H, d, J¼9.6 Hz), 5.29 (1H,
d, J¼9.6 Hz) (H-10 and H-100), 6.22 (1H, s), 6.58 (1H, s),
6.79 (1H, s), 6.96 (2H, s) (aromatic protons); 13C NMR
(CD3OD, 75 MHz) d 19.53, 20.31, 20.64, 20.86, 21.36,
21.94, 23.82, 24.19 (CH3COO and ArCH3), 56.60, 57.01
(OCH3), 63.11, 63.65 (CH2-60 and CH2-600), 69.80, 70.16,
71.30, 71.73, 72.40, 73.39, 75.98, 76.31, 77.11 (CH of Glc),
109.62, 111.42, 112.54, 115.52, 125.10 (aromatic CH),
111.01, 117.54, 121.39, 130.88, 135.39, 139.16, 143.45,
145.33, 159.36, 159.74, 160.11, 161.81, 162.92, 163.72
(aromatic C), 170.83, 171.06, 171.35, 171.72, 172.27,
172.92 (CH3COO), 181.64 (CO-4), 205.58 (CO-16);
FABMS m/z (positive mode) 1163 [MþH]þ, (negative
4.3.5. Preparation of compound 7. To a biphasic solution
of tetra-O-methyl aloeresin H octaacetate (3) (100 mg,
0.086 mmol) in CCl3–CH3CN–H2O, 2:2:5 (12 mL) were
added successively with stirring NaIO4 (1.5 g, 7 mmol) and
RuO2·nH2O (5 mg), and the reaction mixture vigorously
stirred for 2 days at room temperature. After removal of the
precipitated sodium iodate by filtration, and addition of
CH2Cl2 (5 mL), the two layers were separated. The upper
aqueous phase was extracted with CH2Cl2 and the combined
organic layers were dried (Na2SO4) and evaporated under
reduced pressure. The residue was purified by preparative
TLC (EtOAc–CH2Cl2, 1:1) to give 4.0 mg (8.4% yield) of
1
compound 7. TLC (AcOEt), Rf 0.63; H NMR (CD3OD,
300 MHz) d 1.75, 1.94, 1.97, 2.02 (4£3H, 4£s, 4£
CH3COO), 2.35 (3H, s, ArCH3), 3.79, 3.88 (2£3H, 2£s,
2£OCH3), 3.89 (1H, m, H-5’), 4.14 (1H, dd, J¼12.5,
Table 3. Relevant dihedral angles and relative energies of the global minima of aloeresin H aglycone
Conformation
Dihedral angles (values in degrees)
C3C2–C9C14
C13C14–C15C16
C14C15–C16C17
C15C16–C17C18
Energies (kcal mol21
)
1
2
3
4
55.0
113.8
2113.8
253.1
59.7
120.2
2113.9
262.9
73.1
74.5
34.5
34.3
258.4
257.7
261.2
257.6
2123.5
2130.8
2108.8
2110.1
2117.1
2114.1
2116.7
2127.7
130.0
130.3
125.0
125.1
127.4
123.5
126.9
129.3
0.068
1.450
1.098
0.321
0.000
0.561
0.650
0.580
5 (I)
6
7
8 (II)

P. Manitto et al. / Tetrahedron 59 (2003) 401–408
407
Table 4. Relevant internuclear distances in conformers corresponding to global minima of aloeresin H aglycone
˚
Conformation
Internuclear distances (A)
Ha15–H(Me22)
Ha15–H3
Hb15–H(Me22)
Hb15–H13
(O18)H–O16
(O10)H–O1
1
2
3
4
2.285
2.200
2.488
2.469
2.254
2.338
2.262
2.215
2.526
4.764
4.109
2.610
2.459
4.010
3.905
2.600
2.204
2.251
2.274
2.277
2.389
2.458
2.400
2.272
2.535
3.749
3.093
3.083
2.703
2.713
2.679
2.734
2.133
2.130
2.183
2.181
2.165
2.191
2.168
2.148
2.177
3.267
3.257
2.159
2.209
3.314
3.261
2.247
5 (I)
6
7
8 (II)
2.4 Hz, H-6b0 ), 4.26 (1H, dd, J¼12.5, 4.5 Hz, H-6a0 ), 5.04
(1H, d, J¼10.2 Hz, H-100)0, 5.15 (1H, dd, J0¼J00¼9.6 Hz,
H-40), 5.28 (1H, dd, J0¼J ¼9.6 Hz, H-30), 5.94 (1H, brdd,
J0¼J00¼9.6 Hz, H-20), 6.67 (1H, s, Ar-H); relevant NOE
associations, see Figure 2; 13C NMR (CD3OD, 75 MHz) d
20.56 (CH3COO), 22.75 (ArCH3), 56.65, 62.86 (2£OCH3),
63.32 (CH2-60), 69.89, 71.70, 73.84, 76.37, 77.34 (CH of
Glc), 111.76 (CH), 118.33, 131.67, 144.74, 160.87, 163.53
(aromatic C), 174.92 (COOH); 170.95, 171.33, 171.79,
172.21 (CH3COO), 199.78 (CO); FABMS m/z (positive
mode) 577 [MþNa]þ; (negative mode) 553 [M2H]2. Anal.
calcd for C25H30O14: C, 54.15; H, 5.45. Found: C, 54.79; H,
5.36.
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The program Gaussian 9419 at the semi-empirical level
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`
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`
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